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N°6 – Control of air flow to electric heating coils – HSC

For training at PB level, this section and the others will not be studied.

Electric heating coil Source Acim Jouanin

If the air handling unit is equipped with electric heating coils, the french safety regulations concerning fire risks and panic in buildings frequented by the general public, specified in article CH37:

French electric heating coils are thus fitted with:

  • A double safety thermostat, called “Klixon double”. The first ceiling value (approx. 60 [°C] (140°F) has an automatic re-set (the coil can start up if the temperature lowers) and the second ceiling value (75 [°C] (167°F) has a manual re-set.
  • A servo-control system of the fan

Note: a better protection than the simple servo-control of the fan can be obtained by subordinating the function of the electric coil to the effective presence of air flow. This solution will be studied in the next section.


What would be the risk of letting an electric heating coil function without any air flow?

We can imagine that the heating resistance could become “red hot” like an electric hot plate that has been left on in the kitchen.
If the electric heating coil does not stop (by means of the activation of a thermostat monitoring overheating), there is a risk that the inducted radiation becomes a source of fire outbreak.

In addition to the french safety regulations against fire and panic risks in buildings frequented by the general public, article CH 38 specifies that a smoke detector at the outlet of the AHU must be fitted, and, downstream from the filters, a metallic damper activated by the smoke detector for:

  • Air handling units processing more than 10 000 [m³/h] ,
  • Air handling units supplying sleeping accommodation premises,
  • All air handling units linked to one or more distribution networks common to these units, and processing more than 10 000 [m³/h].